This invention relates to the field of recording or imaging systems and more particularly to an intensity controller for a laser, computer-output-microfilm (COM) plotter.
Lasers, because of their narrow beam width, low beam divergence, and high energy density, have found wide application in image recording upon photo-responsive media. U.S. Pat. No. 3,858,004, issued Dec. 31, 1974, and U.S. Pat. No. 4,197,560, issued Apr. 8, 1980, are two examples of the use of modulated laser beams for recording latent images on a xerographic photo-receptor in a facsimile printer. The laser in each of the above mentioned patents is scanned across the photo-receptive surface by a mirror while being intensity modulated in order to produce scans of light areas, dark areas, and half-tone areas. When one scan of the laser is complete, a subsequent scan is performed in close proximity to the previous one. The sequential reproduction of all the scans creates a copy of a facsimile transmitted document very much like a television picture is reproduced. In order to make a good reproduction of the facsimile transmitted document, the intensity level of the laser, between the lightest printing level, and the darkest printing level must be within the dynamic range of the photo-receptor. Therefore, it is necessary to provide such printers with an intensity control to set the intensity of the printing or writing beam to a predetermined operating point within the dynamic range of the photo-receptor about which the intensity may be modulated.
The two above-mentioned patents each disclose an intensity controller system which uses a number of neutral density filters for attenuating the laser beam to various levels. These filter elements are mounted within a rotary mechanism which is rotated to supply various degrees of filtering. After filtering, a portion of the beam is split off and directed to a photodiode to produce an output signal proportional to the intensity of the beam. This output signal is compared with one of a number of predetermined levels, each corresponding to different photo-receptor characteristics of the facsimile system. If the intensity proportional signal does not substantially equal the predetermined level selected then, another filter with either a greater or lesser attenuation is subsequently rotated into position in order to provide a filtered beam of the desired intensity. Systems such as these not only require a lot of extra hardware, space and care, but also require long response times in order to switch and stabilize at the appropriate intensity level. The problem with such a system is that it is too slow and too cumbersome to be applied to a high resolution, aperture card, computer-output-microfilm plotter.
U.S. Pat. No. 4,612,555, issued Sept. 16, 1986, and U.S. Pat. No. 4,613,877, issued Sept. 23, 1986, illustrate the application of laser beam scanners to laser standard size page printing systems. The laser used in each of these systems is a solid state laser. Solid state diode lasers are known for having very small beam generating regions within each specially constructed semiconductor diode. Because the laser beam is generated by a small region, instead of a long path of stimulated emission as in a helium-neon laser, the divergence of the coherent beam is considerably greater than that of gas lasers. The problem of such a design is that extra optical components and thus expense would be required in order to achieve the high resolution necessary for an aperture card COM plotter if a solid state laser were used. Further, a solid state laser which emits light of the wavelength required to properly expose the microfilm is not commercially available.
Industry standards require that an aperture card COM plotter have the same or better resolution, upon reproduction of the recorded drawing to full size, as required of a plotter for producing a full size drawing. Industry standards further dictate that a full size drawing generated by the full size plotter have a resolution of at least 200 dots per inch for the maximum size drawing. Since, in an aperture card plotter, for the worst case, a drawing is reduced by a factor of 30-to-1 in order to be recorded on microfilm, a resolution of 6,000 dots per inch or more is required for a COM plotter of this type.
The designs of other known intensity controllers used in laser printers have higher degrees of complexity, and higher numbers of components, yet do not print at sufficient resolutions to be used for an aperture card plotter. Such systems are not instructive with regard to the design of an intensity controller for an aperture card plotter with high resolution, compact, and straightforward optics.
U.S. Pat. No. 3,787,887, issued Jan. 22, 1974, uses optical components which are both spatially large and responsively slow in order to vary the reflection of a device which has a variable polarization. Since a light beam will be almost totally reflected from a medium having a polarization that is orthogonal to that of the incident light beam, the intensity can be adjusted by varying the polarization direction and thus varying the portion reflected of an incident laser beam. It can be seen that in order to operate properly, the laser of U.S. Pat. No. 3,787,887 must maintain a relatively stable polarization orientation of its output beam. If the laser output beam polarization orientation varies significantly with respect to time, this intensity control mechanism is incapable of responding rapidly enough to perform its function. Small, inexpensive gas lasers, which are desirable from cost and beam-width consideration, exhibit randomly changing polarization of their output beams during warm-up, therefore; if plotter operation is required during the warm-up cycle, an intensity controller which is not independent of the light source polarization presents a problem.
Another type of intensity control system shown in U.S. Pat. No. 4,378,490, issued Mar. 29, 1983, uses any type of light source. The intensity control of this patent operates with coherent, noncoherent, polarized, or non-polarized light sources. The problem of this design, however, is that it uses a physically large controllable attenuator. In order to be modulated by the large, controllable attenuator the source beam must be broadened to a similar physical size. This creates a resolution problem because once the beam is greatly broadened, expensive and powerful optics are required to gather the energy back to a spot that is no bigger than the 1/6000 th of an inch, which as described previously is a requirement for a COM plotter of this type. It is therefore desirable to have an intensity controller that does not require a great broadening of the system light beam in order for the controllable modulator and/or attenuator to operate.
U.S. Pat. No. 3,705,758, issued Dec. 12, 1972, shows an intensity controller system which is designed to broaden the spot generated by the laser system by a distributed optical filter. The purpose of the wider spot so formed, is to provide better data reading and data writing capabilities to a thermally sensitive data recording disk. The system of this patent is designed not to produce narrow spots, but to produce wide spots efficiently. To this end, this patented system utilized anti-reflective coatings on all optical surfaces. It is desirable to use anti-reflective coatings to reduce reflective losses in a high resolution system.
Other complex intensity controller systems are disclosed in U.S. Pat. No. 4,629,879, issued Dec. 16, 1986; U.S. Pat. No. 4,562,567, issued Dec. 31, 1985; and U.S. Pat. No. 4,581,617, issued Apr. 8, 1986. In each of these three patents the modulated beam must travel a respective complex optical path before the beam reaches the intensity sensor. Further, these three patents all manifest the problem of unnecessarily complex intensity control devices. It is desirable to keep the optical path and the control apparatus as simple as possible in order to keep costs down.